A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Wires have conventionally been used to connect an IC to a board but this method is increasingly being replaced with a method referred to as “flip-chip bumping”. In flip-chip bumping a pattern is imaged onto a thick layer of resist (i.e. thicker than a layer of resist used in conventional lithography) which is provided on the substrate. The resist is developed and processed such that recesses are formed at predefined locations and solder is then electroplated in the recesses. The resist is then removed leaving solder “bumps” projecting upwards from the uppermost surface of the substrate. In general, the resolution of the lithographic apparatus may be low, since the accuracy with which the solder bumps need to be located is typically around 1 micron (this is a significantly lower accuracy than the accuracy of tens of nanometers that is provided by high resolution lithographic apparatus).
Processes, such as flip-chip bumping, in which thicker layers of resist are patterned typically employ higher doses of radiation to produce the desired pattern. However, increasing the dose of radiation can lead to increased heating of the resist, substrate and/or substrate support table. For example, the temperature of the substrate in the vicinity of the exposed resist can rise excessively and cause local slippage or expansion of the substrate. Moreover, exposing larger areas of the substrate, or indeed the whole substrate, with increased doses of radiation can increase the overall average temperature of the substrate beyond acceptable limits and result in global slippage or expansion of the substrate and possibly large uncorrectable pattern overlay errors.
A method designed to address undesirable heating of a substrate is to immerse the substrate in a “cooling-” or “conditioning fluid” during exposure such that excessive levels of heat generated in the substrate can dissipate to the surrounding fluid. A problem with this method is that it may require significant modifications to conventional lithographic apparatus, which are both costly and time consuming. A further method intended to alleviate problems associated with excessive heating of a substrate is to provide the substrate support table with integral “cooling-” or “conditioning elements” which maintain the substrate to within a desired temperature range during exposure. In this way, excessive heat generated in the substrate is transferred away from the substrate. Typical conditioning elements comprise a series of channels formed in the support table through which a cooling fluid, such as water, is passed continuously during exposure. Problems associated with this method are that it again may require significant modifications to conventional lithographic apparatus, particularly the substrate support table, and that it is complicated and expensive to design and manufacture the support table so as to be watertight and possess the required flatness.
It is desirable to provide, for example, lithographic apparatus and a method which obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.